Our common salvation
Jude introduces his jewel of a letter thus: “Beloved, while I was very diligent to write to you concerning our common salvation, I found it necessary to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints” (v.3).
While affirming that all true believers already are in possession of salvation (because they belong to Christ, being purchased by His blood), Jude holds the same believers responsible to defend and proclaim the Faith in its purity. But why does he call it “our common salvation”? For several reasons.
It is called "koinos," that is, common, because it is shared by all: what I enjoy, you enjoy. It is a salvation coming down from above, prophetically declared, divinely promised, and historically fulfilled in the coming of the Son of God. If salvation were man's doing, then man would invent a salvation which he thinks would be adaptable for him; but actually our salvation, and the knowledge of the same, is a sacred deposit, and is to be guarded by the church (1 Timothy 6:20), which is the pillar and ground of the truth. Thus her prime responsibility to publish it and preserve it for ages to come. All believers are witnesses, and trustees (Isaiah 43:10).
This great salvation is called common, not because it's cheap (far from it!) or because it's found practically everywhere, but because all Christians (the church militant) enjoy its blessings and privileges. To elaborate on just a few of these gifts:
All believers are chosen by the same free and sovereign grace, not because of any human merit or good disposition within them. Ephesians 1:3f, chosen in him before the foundation of the world, that we should be holy and blameless before him. In love having predestinated us unto adoption of sons by Jesus Christ unto himself. The ineffable grace that has reached me has, to the same measure, reached all other Christians.
This is because grace comes to us by Jesus Christ, and obviously, all Christians belong to Him. They have the same Christ, and all hold fast to him, who is the head, they being members of the same body. They confess the same Lord, the same Redeemer, the same Saviour. Exclusively Him, and no other (Acts 4:12; 1 Timothy 2:5).
Christ, owned by God's children, has earned for them all a perfect and eternal righteousness by his obedience and sufferings during his state of humiliation. He is God's righteousness revealed in the gospel and imputed to all believers. I am not any more justified or better justified than any other: we, the whole number of the elect, enjoy the same righteousness (Romans 3:22).
This righteousness Christians have received by faith, as Peter designates it, "like precious faith," (2 Peter 1:1), for though your faith my be stronger than mine, yet it is a saving faith and it is a faith whose object is exactly the same.
The church's faith finds its logical and natural expression in works of love. They obey the same kanon, the same rule, the same Scripture. They don't reject it, or go beyond the doctrine of Christ. (Galatians 6:16). Christians are known as "people of the book," the same body of revealed truth for them all.
Finally, all Christians are incorporated and baptized in the same mystical body (Colossians 2:19; 1 Corinthians 12:13). Though denominations have been and still are a historical reality, yet true Christians are not separated; rather they enjoy a bond of union and communion with the same Lord and with each other (1 John 1:3; John 17:3,21; 1 Corinthians 1:9).
All these aspects, Jude, under divine inspiration, brings them together under one title, "our common salvation."